Martial told Mourinho that he would be leaving for the birth of his second child after starting the first two matches Stateside and his failure to return from Europe afterwards vexed the manager.
The 22-year-old, who has been strongly linked with a move away after his agent claimed Martial wanted to leave for more opportunities, was absent from United's 2-1 International Champions Cup win over Real Madrid on Tuesday.
Taking to Twitter on Wednesday, Martial wrote in French: "Thank you all for your messages".
This, however, might come as a disappointment to the player, who put pen-to-paper on his first professional contract with the Red Devils last December. "Back in Manchester tomorrow".
Romelu Lukaku (left) and Jose Mourinho (right). "He has the baby and after the baby is born - lovely baby, full of health, thank God - he should be here and he is not here", said the Portuguese coach.
The treatment of the Frenchman is in stark contrast to Liverpool's German boss Jurgen Klopp publically supported his squad member Nathaniel Clyne, who left the club's own U.S. tour to prepare for fatherhood. They will re-assemble at Carrington on Friday.
Martial's future with the club was already in question prior to this situation, having seen his playing time drop after Jose Mourinho took over for Louis Van Gaal following the 2015-16 season.
Mourinho and Martial seem to be at loggerheads with each other with the latter now making headlines for his statements on social media. "We can fight for every title but if the manager wants to strengthen the team, it is something between him and the club".
Former Real Madrid boss Zinedine Zidane, who is now without a job, is favourite to take over the reigns at Old Trafford at 4/1 with Betfair and Paddy Power, according to Oddschecker.
"So, they're coming back three days early to try and be available for the team".